Poking through antiques stores while traveling through the Texas Panhandle, Bill Waters stumbled across a tattered old ledger book filled with formulas.He bought it for $200, suspecting he could resell it for five times that. Turns out, his inkling about the book's value was more spot on than he knew. The Tulsa, Okla., man eventually discovered the book came from the Waco, Texas, drugstore where Dr Pepper was invented and includes a recipe titled "D Peppers Pepsin Bitters.""I began feeling like I had a national treasure," said Waters, 59.Dr Pepper's manufacturer says the recipe is not the secret formula for the modern day soft drink, but the 8 1/2-by-15 1/2 inch book is expected to sell between $50,000 to $75,000 when it goes up for auction at Dallas-based Heritage Auction Galleries on May 13."It probably has some of the original concoction on its pages," Waters said..